The present invention relates to electroconductive composite ceramics. More particularly, the present invention relates to improved electroconductive composite ceramics which have excellent thermal shock resistance, mechanical strength and chemical resistance property and possess stable electroconductivity-temperature characteristics at a high temperature.
Metals, electroconductive resin composite materials, electroconductive ceramics, electroconductive ceramic composite materials and the like are known hitherto as electroconductive materials. However, these electroconductive materials have drawbacks inherent to themselves. Metals are narrow in a range of selecting specific resistance and poor in heat-resistance. Electro-conductive resin composite materials have such an advantage that the temperature characteristics of specific resistance, thermal shock resistance and mechanical strength can properly be adjusted according to selection of the materials, but their low heat-resistance seriously limits the extent of their application. Electroconductive ceramics are limited in the range of their applicable temperature because of their non-linear electroconductivity to temperature and moreover they are poor in thermal shock resistance and mechanical strength. The thermal shock resistance of electroconductive ceramics can be improved to a certain degree when they are processed to electroconductive ceramic composite materials. However, the electroconductive ceramic composite materials are equivalent to the electroconductive ceramics in respect of low mechanical strength and non-linear electroconductivity to temperature and become narrow in a range of selecting specific resistance than the electroconductive ceramics.
Illustrative of the electroconductive ceramics are ceramics for heater elements such as silicon carbide, zirconia and lanthanum chromite. Illustrative of ceramic materials for thermister are those of MnO--CoO--NiO series utilizable at a low temperature around 200.degree. C, those of ZrO.sub.2 --Y.sub.2 O.sub.3 series utilizable at a high temperature around 1000.degree. C and those of spinel series such as CoAl.sub.2 O.sub.4, NiAl.sub.2 O.sub.4, MgAl.sub.2 O.sub.4 and MgCr.sub.2 O.sub.3. However, these are poor in thermal shock resistance and mechanical strength and undergo oxidation or reduction by the influence of an ambient atmosphere at a high temperature to permit change of resistance value whereby their stability on actual use is badly influenced. Especially, in the case of ceramic materials for high temperature thermistor, they have such a shortcoming that the resistance value is too small, for example, at a temperature as high as 1000.degree. C or more.
Now, it has been found that composite ceramics composed of the conglomerate low thermally expansive ceramics and an electroconductive substance exhibit practically excellent properties and are utilizable in a wide variety of fields.